Optics in the metro

Wavelengths, particularly in the context of dense wavelength division multiplexing (DWDM), have a radically different relationship to users in the metro than in the backbone. In the long haul, wavelengths are transparent to users, because there are layers of switches and protocols. All customers see is end-to-end service.

Not so in the metro. In the metro, wavelength reliability and performance can be extremely visible-–to the point where a carrier may fail to deliver on a SLA. Failure here means money lost in the form of paybacks, or worse, irritated customers ripe for poaching by competitors.

READ OUR AUGUST 6 SPECIAL REPORT MFN woes are  metro indicator by Liane H. LaBarba

A lot can go wrong in the metro. When was the last time you noticed that a trunk between Atlanta and Dallas was down for service? When was the last time you noticed service was out to your home?  Unless you happen to work in a network operations center (NOC), you only directly experience local outages.

The metro and the local loop are the weakest links because of the basic nature of networks. The physical transport layer-–fiber, amplifiers, transmitters, receivers-–is much more visible as the network fans out toward the customers. There is simply less route redundancy, protection and capacity. As a result, the metro is vulnerable to challenges and issues hidden in the depths of long-haul networks-–all listed in the fine print of customers' service level agreements (SLAs).

The challenge in the metro is to cost-effectively put reliability and flexibility into the local network. Otherwise, metro service providers will never be able to live up to the SLAs customers consider standard to network services contracts.

The solution is optical wavelength management.

DWDM An optical technology used to increase bandwidth over existing fiber optic backbones. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. --from Webopedia READ MORE AN IEC WEB PROFORUM Dense Wavelength Division Multiplexing (DWDM) tutorial READING MATERIAL From our Archives

DWDM

DWDM is critical for metropolitan area networks. It provides bandwidth; it is cost effective; it creates new revenue opportunities. Realizing these benefits and meeting stringent SLAs requires the adoption of optical wavelength measurement and management.

DWDM packs multiple light frequencies (signal channels) onto a single fiber. DWDM is ideal for long-haul communications. It creates additional bandwidth or future expansion capacity with existing fiber. Rather than installing more fiber in the ground, long-haul carriers add more wavelengths to a given fiber. All of the cost is in the electronics (Sonet), and active optics (lasers, receivers, amplifiers) that create the additional channel (wavelength) in an existing fiber ring.

In the metro arena, DWDM has a more complex role. It is still a bandwidth solution that increases the trunk capacity between two tandem central offices. But it is also a more cost-effective, reliable way to deliver service, and, as carriers begin to market wavelengths in bulk or as bandwidth-on-demand, it is the service. One of the attractions of DWDM in the metro is all optical nodes-–such as add-drops-–that allow for the distribution of signals without expensive electrical regeneration.

The appeal of DWDM in the metro is undeniable--lower cost distribution of high capacity bandwidth--but it requires scaling back the reliability mechanisms that made it work in long haul.

The Metro Dilemma 

The very layout of the long-haul topology guarantees reliability. By combining technologies such as Sonet, topologies such as rings and redundant paths, reliability is built into the network.  This combination can detect a failure on a given link, and redirect the traffic on to an alternate link or a different path entirely. All of the problem detection is done at the digital/electronic level (Sonet), and the recovery is a reroute. 

The wavelengths are just dumb pipes. The electronics-–Sonet, Ethernet, SDH--are responsible for the intelligence in the network. The photons are converted back into electronic signals, and techniques such as BERT (bit error rate detection) are used to indicate signal deterioration somewhere in the plant. Alternately, the absence of a signal indicates a catastrophic failure such as a fiber break. Every optical link begins and ends with an electronic termination, along with optical lasers, amplifiers and receivers.

The long-haul model cannot be duplicated in the metro. Metro networks still use Sonet (or its equivalent such as GigE or SDH), rings and redundancy. However, there are just not enough of them to go around, nor is it cost effective to put them in place everywhere. Cost limitations prohibit putting redundant paths and rings everywhere. 

Electrical conversion is avoided whenever possible. Putting Sonet at the end of every link is extremely expensive, and it means that less reliable electronics must be put in vulnerable locations.  These conversion points are costly (requiring both active optics and electronics, as well as power and cooling), unreliable (actives and electronics are much more inclined toward deterioration or failure), and inflexible (adding a wavelength requires adding electronics and active optics at every link point, as well as tuning these elements at every link).

This is the dilemma. The appeal of DWDM in the metro is undeniable--lower cost distribution of high capacity bandwidth--but it requires scaling back the reliability mechanisms that made it work in long haul. Increased capacity, no matter how economical, is of little value to metro service providers if SLAs can’t be honored. Carriers need to deliver bigger bandwidth and better quality service to customers.

Wavelengths can’t be treated like dumb photons that sit under smart electrons.

Wavelength Management and Measurement

The solution to this dilemma is to continuously manage and measure wavelengths. Wavelengths can’t be treated like dumb photons that sit under smart electrons. Rather, DWDM channels must be understood as a network in their own right. In this way,  the wavelengths become a resource that can be optimized and maintained.

Optical wavelength measurement entails putting in-line optical instruments at critical nodes in the network-–switches, add-drops, cross-connects, amplifiers-–and capturing information about each wavelength on a real-time basis.

Sometimes this information needs to be very rich and accurate. Precise data about power, wavelength drift, optical signal to noise ratio can be used locally for real-time applications such as gain balancing, or remotely via communications services for actively and preemptively tracking plant performance.

At other times, this information can be simple and coarse. Lower accuracy power information at remote amplification points can be relayed back to a central point to monitor ongoing operation.

These types of measurements now enable management of the wavelengths. Wavelength failures can be predicted well in advance of Sonet level problems; optical problems can be distinguished from electrical problems. Sonet can tell you when the bits are bad but not why. Problems can be isolated in the network-– which link, which element--much more quickly, without multiple truck rolls.  Ultimately, it becomes possible to flexibly add and drop wavelengths to a fiber remotely, and then properly retune the signal transmission.

Multiple Technologies

Optical wavelength measurement and management will require the deployment of multiple technologies. Today, people tend to think of optical performance monitors as a one size fits all proposition without understanding the different needs in the network. Think of it this way: No one questions the range of different laser needs--LEDs to tunable lasers. The same holds true for optical performance management and measurement.

At the lowest level are optical power monitors. These simple devices must be distributed wherever a low-cost detector is needed. However, they must be coupled to communications links to bring the information back to a central point such as a network operations center. Otherwise they are nothing more than local craft that help a field service operator, but offer no savings or strategic advantage.

At the next level are the different kinds of performance and channel monitors (OPMs and OCMs). These can provide more accurate data on power, wavelength and OSNR, but vary widely on speed and performance. When used locally in a switch or add-drop, they can be used to direct first-order gain balancing, laser locking or attenuation.

The ultimate leverage occurs when the local measurement is linked to distributed wavelength management and reporting capabilities, that in turn connects to a network operating system such as SNMP or TL1. Now the information can be gathered together at a NOC to drive true link and path level management.  This provides health about a given node, and enables link level calculations. For example, by measuring signal strength at two different nodes, it is possible to calculate fiber insertion loss. 

Alternately, examining OSNR via multiple measurement points can be used to determine the overall optical signal regeneration behavior of a link. It also becomes possible to spot active elements in the network (lasers, amplifiers) that are continuously drifting or starting to fail. Combining this all together, before a path fails, carriers can redirect traffic onto a back-up path--even with all optical nodes-–and then rebalance the signals from end-to-end.

Making DWDM Work in the Metro

Metro carriers can deliver wavelengths much more reliably and flexibly, while still enjoying the cost benefits of DWDM. Optical wavelength management provides a cost-effective way to ensure quality of service. It eliminates the need to overbuild for redundancy and to add expensive, high-maintenance electronics. Instead, the wavelength becomes the resource.

The result? Metro service providers deliver more quality bandwidth to customers through a more economically sound network delivery system. Wavelength management and measurement, as a result, becomes key to improving carrier bottom lines. In today’s challenging network environment, there is no other choice but to deliver more, better.
Isadore Katz is President and CEO of Lightchip, Inc. He can be reached at ikatz@lightchip.com.

Visit Lightchip online.

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